US3889155A

US3889155A - Apparatus for calibrating an image dissector tube

Info

Publication number: US3889155A
Application number: US448325A
Authority: US
Inventors: Iii Edwin E Klingman
Original assignee: National Aeronautics and Space Administration NASA
Current assignee: National Aeronautics and Space Administration NASA
Priority date: 1974-03-05
Filing date: 1974-03-05
Publication date: 1975-06-10
Anticipated expiration: 1992-06-10

Abstract

The photosensitive screen of an image dissector tube is illuminated with a light pattern having parallel opposite edges, and a sweep signal is applied to the deflection coils of the tube causing scanning of the pattern in a line perpendicular to the edges and generation of an output video pulse. The sweep signal is in the form of a time variable current whose average rate of change during the scan of the line is a constant and dependent on a settable control circuit. Measurement of the output pulse width permits the setting of the slope control circuit to be changed if the width differs from a standard associated with the tube whereby the scan rate is kept constant despite changes in the deflection sensitivity of the tube.

Description

[ June 10, 1975 APPARATUS FOR CALIBRATING AN IMAGE DISSECTOR TUBE Primary ExaminerMaynard R. Wilbur Assistant ExaminerRichard E. Berger [75] inventor: Edwin E. Klingman, III, Huntsville,

Ala.

Attorney, Agent, or FirmGeorge J. Porter; L. D. Wofford, Jr.; John R. Manning [73] .Assignee: The United States of America as ABSTRACT The photosensitive screen of an image dissector tube represented by the Administrator of the National Aeronautics and Space Administration, Washington,

Mar. 5, 1974 22 d is illuminated with a light pattern having parallel op- 1 l e posite edges, and a sweep signal is applied to the de- [21] Appl. No: 448,325 flection coils of the tube causing scanning of the pattern in a line perpendicular to the edges and genera- 52 us. c1. 315/369; 315/10; 315/367; of an output pulse The Sweep slgnal 315/387 the form of a t1me varlable current whose average rate HOlj 31/26 315/369, 10, 367. 387;

[ Int C1 of change during the scan of the line is a constant and dependent on a settable control circuit. Measurement [58] Field of Search of the output pulse width permits the setting of the l78/DIG. 29

slope control circuit to be changed if the width differs from a standard associated with the tube whereby the scan rate is kept constant despite changes in the deflection sensitivity of the tube.

[56] References Cited lJNITED STATES PATENTS 3,358.184 2/1967 Vitt, 315/369 X 10 Claims, 2 Drawing Figures 1 APPARATUS FOR CALIBRATING AN IMAGE DISSECTOR TUBE ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or thereafter.

BACKGROUND OF THE INVENTION This invention relates to apparatus for calibrating image dissector tubes, and more particularly digitally controlled image dissector tubes.

An image dissector tube is a non-storage type of TV camera tube in which an electron image of the scene being televised forms on a photoemissive screen and is drawn down the tube, as a focused entity, by accelerating anodes towards a shield having an aperture effecting passage of photoelectrons in that portion of the image comprehended by the aperture. Such portion corresponds to an elemental area of the screen termed the selected elemental area." and its location thereon is determined by the level of current flowing in deflection coils associated with the tube.

The electrons passing through the aperture of the shield form a beam that is amplified in a photomultiplier section producing an output signal whose amplitude is proportional to the intensity of light incident on the selected area of the screen. Moving the location of the selected area on the screen is achieved by shifting the entire electron image in the plane of the shield by a change in the level of current flowing in the deflection coils. Generally, such coils have two sets of windings, one controlling the x position of the image, and one controlling the y position. By supplying the proper level of current in each winding, the selected area of the screen can be positioned at any location.

In a recent development, digitally controlled operation of image dissectors tubes has been introduced. In such operation, the inputs to the deflection coils are determined by the contents of a pair ofx, y digital storage registers interfaced with the respective coils through digital-to-analogue converters. The x, y position of the selected elemental area is established by the respective numbers in the registers. Scanning of a line y constant on the screen is carried out by sequentially moving the selected elemental area along such line. This is achieved by maintaining the contents of the y register and by indexing the x register by an increment and at a rate compatible with the scan required. The input to the x winding is thus a staircase-shaped current pulse. If the step height is the current increment Ai, if the period of the steps is At, and if the scan is displaced a distance Ax due to the change in current i, then it follows that the rate at which the scan moves, Ax/ A! is as follows:

The parameter (Ax/ Ai), termed the deflection sensitivity, is the displacement of the scan per unit change in current; and the parameter {Ai/ AI) is the time rate of change of the current. In a given tube, and under fixed external conditions, both parameters will have values determined by a calibration process, and may remain constant over a period of time. The value of the parameter (Ai/ At) is the average slope of the staircaseshaped current pulse and may be changed by changing the number Ai by which the x digital storage register is incremented (assuming a fixed clock period At). As to the deflection sensitivity, it depends on such factors as the physical relationship between the x-windings and the tube, the size of the aperture in the shield, tube geometry and the magnitude and direction of stray electromagnetic fields in the tube. Consequently, its value is not directly determinable but is established as a consequence of the calibration process.

Conventionally, calibration of a tube involves focusing a known calibration light pattern on its screen, and while inspecting a reproduction of such pattern on a stabilized monitor, adjusting the deflection coils of the tube and selecting the slope of the sweep signal until the reproduced pattern is undistorted. This process establishes the scan rate Ax/ At which remains fixed as long as the factors influencing such rate remain unchanged. With aging of the tube and the'circuitry associated therewith, or when operation occurs in an environment in which stray electromagnetic fields in the tube are different from their values during calibration, the scan rate will change stretching or contracting the reproduced image in the direction of the scan. When this occurs, recalibration following the above described process is required.

In addition to being a time consuming operation, the conventional recalibration process has the disadvantage of interfering with the normal operation of the tube. Furthermore, calibration under laboratory conditions with one set of stray electromagnetic fields is ineffective in securing fidelity of reproduction when operation is under field conditions wherein stray fields are different.

SUMMARY OF THE INVENTION The present invention has for its primary object the provision of apparatus for calibrating an image dissector tube which does not suffer from the above described disadvantages.

Briefly, the apparatus of the present invention senses changes in the deflection sensitivity and causes an inverse change in the slope of the sweep signal such that the deflection rate remains constant. This result is achieved by apparatus that illuminates the screen of the tube with alight pattern having parallel opposite edges, and that applies to the deflection coils of the tube a sweep signal producing a scan through the light pattern in adirection perpendicular to the edges and developing a video signal in the form ofa pulse. The sweep signal causing the pulse is in the form of a time variable current whose average rate of change is constant and dependent on a settable slope control circuit. Measurement of the pulse width permits the setting of the slope control circuit to be changed if the pulse width differs from a standard associated with the tube.

If the period of the sweep signal is fixed, the pulse width will be directly proportional to the deflection rate and will have a predetermined value when the tube is in calibration. Changes in the pulse width arise from changes in the scan rate due, either to a change in the deflection sensitivity, or to a change in the slope of the sweep signal. Specifically, if the width has decreased, the scan rate will have increased, stretching the reproduced image of the scene being televised in the direction of the scan absent the automatic means of the present invention which responds to a decrease in video pulse width by a reduction in the slope of the sweep signal that slows down the scan rate. Conversely, if the pulse width exceeds the standard, the scan rate will have decreased distorting the reproduced image by contracting it. The automatic means of the present invention responds to this situation by increasing the slope of the sweep signal thus speeding up the scan rate.

In accordance with the present invention, calibration is thus carried out within the time required to complete the scan of one or two lines, i.e., within a fraction of the frame or field times. In addition to being carried out rapidly, calibration can be done in-situ without substantial interference with normal tube operation since the required light pattern can be developed from a source out of the field of view of the optical system of the tube and superimposed on the screen of the tube.

An embodiment of the invention is illustrated by way of example in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram of apparatus according to the present invention for calibrating a digitally controlled image dissector tube; and

FIG. 2 is a composite time diagram illustrating the relationship between the parameters of the deflection sensitivity and slope of the sweep signal on the pulse width for various values of the parameters.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, reference numeral designates apparatus according to the present invention for calibrating a conventional image dissector tube 20. As is well known, such a tube comprises an envelope 11 containing photosensitive screen 12, an internal apertured shield 9, deflection coils 13 and a photomultiplier section 14. Light from a scene being televised is focused by an optical system, indicated schematically at 17, onto screen 12 on wich an electron image of the scene is formed. By means of conventional anodes (not shown), the image is drawn, while being maintained in focus, from the screen towards the apertured shield. The aperture 9A in the shield permits those electrons in the portion of the image comprehended by the aperture to pass through the shield in the form of a beam 3 that originated from an elemental area of the screen (termed the selected elemental area) 12A whose position is determined by the level of current in deflection coils 13. The beam is amplified in section 14% to provide a video signal representative of the intensity of light falling on the selected elemental area.

The coils 13 are organized into horizontal and

vertical windings

13H and 13V for controlling the horizontal and vertical coordinates, respectively, of the selected area. Scanning of the screen 12 to establish a raster is achieved by synchronizing changes in the level of current 13V. through the windings 131-1 and 13V.

Turning now to apparatus 10 for calibrating tube 20, such apparatus includes means 21 for generating a calibration signal, light source means 22, settable

slope control circuit

23H or 23V, current generator 24H or 24V, and automatic means 25. Means 21 for generating a calibration signal includes clock 26 producing periodic clock pulses a and calibration control circuit 27 responsive to the output of the clock for periodically producing a calibration signal b and other control signals to be described below. The periods of these control signals and their durations will be explained below.

In one form of the invention, light source means 22 includes a selectively operable light source in the form of light-emitting-diode 21$ energized when relay 29 is closed by calibration signal b, and mask 16 interposed between source 211 and screen 12 of tube 20. Opening 15 in mask 16 is congruent with the desired raster and is defined by a pair of spaced parallel vertical edges 18, I9 and a pair of spaced parallel horizontal edges. The aspect ratio of the raster may have any desired values. As seen in FIG. 1, source 28 is out of the field of view of the optical system 17 and, when energized, floods screen 12 with light independently of light from the scene being televised.

Current generator IMH comprises digital horizontal scan register 30H whose contents constitute the x coordinate of the selected elemental area, digital-toanalogue converter 31H for converting the contents of register 30H into the required current flowing in winding 131-1, and horizontal scan control circuit 32H for controlling the changes in the contents of register 30H. The y coordinate of the selected elemental area is determined by the contents of the vertical scan register 30V which is converted into the required current flowing in winding 13V by converter 31V of the vertical current generator 24V. Vertical scan control circuit 32V controls changes in the contents of register 30V.

Scan control circuits 32H and 32V, as their names imply, control the scanning of screen 12, synchronization being achieved by way of timing pulses a derived from clock 26. In its normal mode of operation, each of circuits 321-] and 32V causes the respective registers 301-1 and 30V to be incremented by an amount and at a rate dependent on the type of scanning involved. For example, if the normal scan is to be a series of sequential horizontal lines on screen 12, the required output of

converters

31H and 31V are periodic staircase current pulses. The period of the pulses produced by converter 31H is the time required to scan one horizontal line on screen 12 from one vertical edge 18 of opening 15 in mask 16 to the other vertical edge 19, or vice versa. Converter 31V, on the other hand, produces pulses whose period is the time required to complete a frame (i.e., scan all of the horizontal lines from the top to bottom edge of opening 15, or vice versa), or a field of the raster depending on the type of scanning involved.

Finally, automatic means 25 of apparatus 10 includes gate 33 which passes the video signal from section 141 to analogue-to-digital converter 34 in response to calibration signal b, and computing components in the form of pulse width computer 35, correction computer 36, adder circuit 37 and associated gates and controls. The operation of automatic means 25 is described in connection with the calibration mode of operation of scan control circuits 32H and 32V.

When calibration control circuit 27 develops a calibration signal b, it also develops either a horizontal calibration mode signal or a vertical calibration mode signal, depending upon how circuit 27 is programmed. This point is discussed in detail after the operation of apparatus 10 in a calibration mode is described. Assuming, for convenience, that control circuit 27 has produced a horizontal calibration mode signal, horizontal scan control circuit 321-1 responds by presetting scan register 30H with a number representing coordinate x and by presetting slope control circuit 23H (which is an index register) with a member representing the incremental horizontal current step Ai Simultaneously, vertical scan control circuit 32V responds to a horizontal calibration mode signal by presetting scan register 30V with a number representing coordinate y and by presetting slope control circuit 23V (which is an index register like register 23H) with a number representing the incremental vertical current step which, in general has the same value as Ai With registers 301-1 and 30V thus preset, the scan will start from an initial selected elemental area having the coordinates x y As explained below, these coordinates are chosen to locate the start of the scan at a point beyond one of the

vertical edges

18 or 19 of opening and outside the bounds of the raster. Preferably, the value y is about halfway between the top and bottom horizontal edges of opening 15.

On the next clock pulse applied to control circuit 32H, the latter starts a routine by which the contents of index register 23H are iteratively added to the contents of register 30H in synchronism with the clock pulses a causing a horizontal line y y to be scanned. The routine ends after n clock pulses (corresponding to the time required to scan a horizontal line) with the final selected elemental area at the end of the routine being located at x y H on the opposite side of screen 12 from the location of the preset selected elemental area at a point beyond the other of the

vertical edges

18 or 19 of opening 15 and outside the bounds of the raster.

At the end of the predetermined number of clock pulses, the scan register 30H is reset to its preset value x scan register 30V is incremented to y 1 and the above routine is repeated causing the next horizontal line to be scanned. This cycle of events may be repeated a number of times until the termination of the horizontal calibration mode signal whereupon circuits 32H and 32V resume their normal mode of operation and scanning of the raster reverts to normal.

During a routine described above, the output of converter 31V is a constant current while the output of converter 31H is a staircase current pulse similar to any of the pulses 38-0 to 38-4 shown above the time axis in FIG. 2. Each pulse is shown with only eight steps to facilitate illustration and explanation of the invention. The average slope S of each current pulse is the average rate of change of the sweep signal current flowing in winding 13H during the scan of one line. Since S (Ai/ At), this slope is a parameter adjustable at the end of the scan of a line by changing the increment Ai, the number contained in index register 23H, before the next scan begins.

The displacement of the scan due to a staircase current pulse is shown by the curves below the axis in FIG. 2. The average slope of each curve 39-0 to 39-4 represents the scan rate R (Ax/ At) during the scan of a single line. As seen from equation (1), the scan rate is dependent on two parameters; the slope S of the sweep signal controlled by the contents of register 23H, and the deflection sensitivity K (Ax/ Ai) which is subject to change due, for example, to a change in stray electromagnetic fields in the tube.

As a consequence of the displacement of a scan through the pattern of light on screen 12 of width W as defined by the spaced,

parallel edges

18 and 19 of opening 15 in mask 16, the tube produces video pulses like that shown at 40-0 to 40-4, each of duration AT (W/R). Since the width of the pattern is fixed, any change in the duration of the video pulse from the scan of one line to another will have arisen because of a change in the scan rate. Hence, monitoring the duration of the video pulse produced during consecutive routines described above will provide an indication of changes in the scan rate, which, it will be recalled, must remain constant if the reproduced image is to be distortion-free. Such changes from a scan rate established when the tube is in calibration can be used to modify the single most easily adjusted parameter, the slope of g the sweep signal so that on the next scan, the scan rate is restored to the value it had on calibration. This, briefly, is the result achieved with automatic means 25 of the present invention.

Before describing the operation of the automatic means, reference is made once more to FIG. 2 to illustrate the theory of operation involved. Assume that pulse 38-0, curve 39-0 and pulse 40-0 are associated with a tube known to be in calibration the slope of pulse 38-0 is S =+l by reason of the units chosen; the slope of curve 39-0 is R, +1 by reason of the units chosen; and the pulse duration is ATo 3 clock pulse intervals. Assume now that the deflection sensitivity increases from K, 1 unit (whose dimensions are meters per amperes) to K 3/2 units before the next line is scanned. Since the contents of register 23H remains the same, the slope of the next sweep current pulse 38-1 will remain S +l,but the scan rate will change from R, =+1 to R =+l.5. The consequence of the change in the deflection sensitivity and the consequential change in scan rate is manifested by a change in the video pulse duration from AT, 3 units to AT 2 units. Since the duration is less than the standard associated with the tube when it is in calibration, the contents of register 23H must be increased. Intuitively, the increase should be inversely related to the decrease in the pulse duration. That is, the new slope of the sweep signal should be old In the above example, the new slope S should be (2/3) l 2/3. Pulse 38-2 has this slope; and with this slope and a deflection sensitivity K 3/2 (unchanged from the previous line scan), the scan rate returns to .unity (R K S) and the pulse duration of pulse 40-2 is 3 units. The tube is once again in calibration.

Assume now that the deflection sensitivity decreases from 3/2 units to 9/8 units. Since the contents of register 23H remain unchanged, the next sweep current pulse 38-3 will have the same slope S as the last current pulse 38-2. But since the deflection sensitivity has changed, the scan rate changes from unity to As seen in FIG. 2, the resultant video pulse 40-3 is 4 units duration. Applying equation (2), the new slope 8., should be (4/3) as (8/9). Pulse 38-4 has this slope; and with the same deflection sensitivity K, 9/8, the resultant video pulse 40-4 will have the calibration duration, 3 units.

Turning now to details of the operation of automatic means 25, the calibration signal b opens gate 33 applying the video output of tube 20 to converter 34, the duration of the video pulse produced during calibration being digitally computed in computer 35 whose output is a number representative of the pulse duration. In the horizontal calibration mode of operation, gate ll is open to permit insertion of the contents 8,, of register 23H to be inserted into correction computer 36 which performs the operation indicated:

AS [(A O) 130111 The number S which is the current contents of register 23H, and the number AS which has both sign and magnitude, represents the increment by which the current contents of register 23H must be changed, are applied to adder 37. Add signal generated by calibration control circuit 27 occurs between line scans and transfers the new sum S AS via gate 42, enabled by the horizontal calibration mode signal, to register 231-1. The next scan is thus carried out with the new number in register 23H causing the sweep current to have a new slope.

Depending upon the magnitudes of the increments Ar and M, and the accuracy of the

converters

31H, 31V and 34, horizontal calibration can be achieved in only a few lines. Moreover, once horizontal calibration is achieved, the circuit 27 can be programmed to switch to a vertical calibration mode and vertical calibration can be achieved equally as rapid.

MODIFICATIONS Regardless of the calibration mode, i.e., vertical or horizontal, the present invention requires, among other things, that the light source means 22 provides a partic ular type of light pattern on screen 12, and that the scan in the calibration mode be particularly related to the light pattern. First of all, the pattern can be developed in a manner different from the source/mask arrangement shown in FIG. 1. For example, it could be projected by a suitable lens system onto the screen since the pattern only need have opposite parallel edges, and the scan direction during calibration should preferably be normal to such edges. Moreover, the size of the pattern need not match the raster size as developed during the normal mode of scanning, but the optimum relationship appears to be equality. Finally, the light source itself can be other than a light emitting diode.

Having established the spacing between'the clearly] defined edges of the light pattern, it is also essential to insure that the scan line through the pattern start and end just beyond the edges of the pattern. In this respect, a horizontal scan line in the calibration mode will normally be slightly longer than a horizontal scan line in normal operation.

By reason of the discrete noncontinuous steps in the sweep signal, and the accuracies of conversion of the digital signals to analogue and vice versa, perfect calibration may not be achieved after the scan of a single line; and for this reason the calibration signal should be long enough to permit the scan of several lines of the light pattern. Furthermore, the time from one calibration signal to the next is dependent on the circumstances, it being possible to revert to the calibration mode as frequently as necessary, possibly at the frame frequency or every few seconds, all without significantly interfering with the normal use of the tube.

While the automatic means shown in the drawing computes the value AS to be added to the contents of the horizontal or vertical index register depending on whether the apparatus is operating in its horizontal or vertical calibration mode, it is of course possible to merely compute the new slope directly from the rela tionship shown in equation (2) thus eliminating the need for adder 37.

What is claimed is:

1. Apparatus for automatically calibrating a TV camera tube having a photosensitive screen on which an electron image of the scene being televised is formed, and a system for developing a video signal from a beam of electrons originating from the screen at a selected elemental area whose position depends on the level of current flowing through deflection coils associated with the tube, the apparatus comprising:

a. means generating a calibration signal;

b. light source means responsive to the calibration signal for illuminating the screen with a light pattern having parallel opposite edges;

c. a settable slope control circuit;

d. a current generator responsive to the calibration signal for supplying to the deflection coils a sweep signal that sequentially moves the location of the selected elemental area along a line crossing the pattern in a direction perpendicular to the edges and extending therebeyond whereby the pattern is scanned and the tube produces a video pulse;

e. the sweep signal being in the form of a timevariable current whose average rate of change is constant and dependent on the setting of the slope control circuit; and

f. automatic means responsive to the duration of the video pulse for changing the setting of the slope control circuit if such duration is different from a standard associated with the tube.

2. Apparatus according to claim 1 wherein the light source means includes a light source responsive to the calibration signal for flooding the screen with light and a mask adapted to be interposed between the source and the screen, the masking having an opening defined by pairs of opposite, parallel edges congruent with the desired raster thereby limiting the lighted area of the screen to such raster.

3. Apparatus according to claim ll wherein the automatic means sets the slope control circuit so as to increase the slope when the duration of the pulse is less than the standard and to decrease the slope when the duration is greater than the standard.

4. Apparatus according to claim 3 wherein the slope selector circuit includes an index register, and the current generator includes a storage register whose contents establish one coordinate of the selected elemental area, a digital-to-analogue converter for converting the contents of the storage register to a level of current flowing in the deflection coils, and control means for periodically incrementing the storage register by the contents of the index register.

5. Apparatus according to claim 4 wherein the automatic means includes means for changing the contents of the index register, subsequent to a scan, by the quantity S [(AT/ AT ]l] where S is the contents of the index register during the scan, AT is the duration of the video pulse produced by the scan, and AT is the pulse duration when the tube is in calibration.

6. Apparatus according to claim 4 wherein the automatic means includes an analogue-to-digital converter for converting the video pulse to a digital signal, and means responsive to the last mentioned digital signal for computing the duration of the video pulse, AT.

7. Apparatus according to claim 6 including means for computing the quantity S [AT/ AT,, 1] at the end of the scan, where S is the contents of the index register during the scan, and AT is the pulse duration when the tube is in calibration; and means for adding such quantity to the contents of the index register at the end of the scan.

8. The combination of the apparatus according to periodically generating a calibration signal.

Claims

1. Apparatus for automatically calibrating a TV camera tube having a photosensitive screen on which an electron image of the scene being televised is formed, and a system for developing a video signal from a beam of electrons originating from the screen at a selected elemental area whose position depends on the level of current flowing through deflection coils associated with the tube, the apparatus comprising: a. means generating a calibration signal; b. light source means responsive to the calibration signal for illuminating the screen with a light pattern having parallel opposite edges; c. a settable slope control circuit; d. a current generator responsive to the calibration signal for supplying to the deflection coils a sweep signal that sequentially moves the location of the selected elemental area along a line crossing the pattern in a direction perpendicular to the edges and extending therebeyond whereby the pattern is scanned and the tube produces a video pulse; e. the sweep signal being in the form of a time-variable current whose average rate of change is constant and dependent on the setting of the slope control circuit; and f. automatic means responsive to the duration of the video pulse for changing the setting of the slope control circuit if such duration is different from a standard associated with the tube.

3. Apparatus according to claim 1 wherein the automatic means sets the slope control circuit so as to increase the slope when the duration of the pulse is less than the standard and to decrease the slope when the duration is greater than the standard.

5. Apparatus according to claim 4 wherein the automatic means includes means for changing the contents of the index register, subsequent to a scan, by the quantity S ( ( Delta T/ Delta To -1) where S is the contents of the index register during the scan, Delta T is the duration of the video pulse produced by the scan, and Delta To is the pulse duration when the tube is in calibration.

6. Apparatus according to claim 4 wherein the automatic means includes an analogue-to-digital converter for converting the video pulse to a digital signal, and means responsive to the last mentioned digital signal for computing the duration of the video pulse, Delta T.

7. Apparatus according to claim 6 including means for computing the quantity S ( Delta T/ Delta To - 1) at the end of the scan, where S is the contents of the index register during the scan, and Delta To is the pulse duration when the tube is in calibration; and means for adding such quantity to the contents of the index register at the end of the scan.

8. The combination of the apparatus according to claim 1 with an image dissector tube having a screen.

9. The combination of claim 8 including an optical system for directing light from the scene being televised onto the screen, and wherein the light source means includes a light source responsive to the calibration signal for flooding the screen with light and a mask adapted to be interposed between the source and the screen having an opening congruent with the desired raster, the light source being out of the field of view of the optical system.

10. The combination of claim 9 including means for periodically generating a calibration signal.